The use of ammonium nitrate (AN)--based gas generating compositions are well known, particularly for use in inflating devices such as air bag inflators. One of the problems with AN-containing compositions is the existence of a melt zone at the burning surface during the combustion process. This melt zone is positioned between the unburned AN material and the flame front. Melting of the AN at the flame front interface prior to combustion adversely affects the combustion of the overall composition, i.e., it lowers the burning rate of the gas-generating composition, since the melting and associated melt phase reactions are endothermic in nature. AN-based mixtures also exhibit a higher than ideal pressure exponent, particularly at low pressures. Generally, it is desired to have pressure exponents in the range of about 0.4 to 0.6. Pressure exponents above 1 can cause uncontrolled burning and over pressurization in choked motor designs. AN-based mixtures generally operate satisfactorily at high pressures. However, at lower pressures, AN-based mixtures have a tendency to extinguish themselves prior to complete combustion especially under regressive pressure conditions. With extinguishment, a large residue of uncombusted material remains, thereby adversely affecting the yield and economy of AN-based mixtures.
A prior art solution to the problems noted above (i.e. low burning rate and high exponent) with respect to AN-based compositions is the use of catalysts such as chromium or nickel salts. While these catalysts may effectively increase the burning rate, they produce toxic exhaust products that are generally not found acceptable in automotive air bag applications.
Consequently, a need has developed to provide improved gas generating compositions using AN or similar oxidizers which burn effectively at low pressures, e.g., have a desirable pressure exponent, and have a high burning rate. The present invention solves this need by using these types of compositions with an aluminum powder made by electro-explosive techniques, hereinafter "exploded aluminum powder". Using the exploded aluminum powder unexpectedly both increases the burning rate and lowers the pressure exponent for AN-based compositions and compositions similar thereto.
In the prior art, aluminum powder has been used as a fuel in gas generating compositions. Reissue Patent No. 25,692 to Cook et al. relates to an improved slurry explosive composition and, more particularly, to an explosive composition comprising ammonium nitrate, a heat producing metal such as aluminum, and water.
U.S. Pat. No. 2,589,532 to Byers discloses an explosive composition and, more particularly, but not exclusively, relates to alkali nitrate compositions in which ammonium nitrate is the predominating explosive ingredient. The Byers patent involves the discovery that when an atomized metal, aluminum, in the lower micron sizes is used in explosives, and more particularly in nitrate explosives, either with the metal particles as nuclei for the nitrate crystals, or as an exterior coating for the same, or are otherwise then mixed with the nitrate, there results an explosive which may be detonated by an ordinary no. 6 commercial blasting cap and one which shows good propagation characteristics.
U.S. Pat. No. 3,580,753 to Griffith discloses explosive mixtures based on trimethylolethane trinitrate and an inorganic nitrate, having a high rate of detonation and good sensitivity due to the incorporation of aluminum particles in the explosive.
None of the prior art discussed above teaches or suggests the use of exploded aluminum powder as part of a gas generating composition nor the unexpected benefits of increased burning rates and lowered pressure exponents.